Optical networks are used as high-capacity transmission networks. As one scheme of optical network, a passive optical network (PON) is known in which optical signals transmitted in bursts from multiple optical network units (ONUs) are combined by a star coupler and received by a single optical line terminal (OLT).
A PON to which time division multiplexing (TDM) is applied is known as TDM-PON in which downstream signals from the OLT to the ONUs are transmitted according to TDM and upstream signals from the ONUs to the OLT are transmitted according to time division multiple access (TDMA). Since the wavelength used in the upstream transmission system of TDM-PON is 1.3 μm band, the fiber loss is greater than transmission using 1.55 μm band and signals are likely to be deteriorated.
Further, in the upstream transmission system of TDM-PON, the optical power is significantly deteriorated due to splitting by the optical splitter (optical coupler). Thus, it is further difficult to increase the number of splits and/or to extend the transmission distance. Generally, an increase of the number of splits and an extension of the transmission distance have a trade-off relationship since the number of splits and the amount of loss/deterioration at the optical splitter have a substantially inverse relationship. An OLT is also known that causes light output from a fiber array to be received by a PD using a lens array and a condensing lens (see, for example, Cheng, Ning, et al, “Large Splitting and Long Reach Passive Optical Networks with Mode Coupling Receivers,” ECOC, September 2010).
However, in the conventional technology described above, the optical loss of signals received by the OLT differs for each ONU if the distance to the OLT differs for each ONU. Thus, it is difficult to keep the power of light that is received at a photo diode of the OLT within a given dynamic range. Consequently, light from the ONUs cannot be accurately received.
To cope with this problem, an optical pre-amplifier may be provided in each ONU to independently amplify the light from the ONU at the ONU. However, this results in a larger and more expensive ONU. Further, at the OLT, it is difficult to control the gain of the optical amplifier provided in each ONU.
Alternatively, an optical post-amplifier may be provided in the OLT to amplify, at the OLT, the light transmitted according to TDMA from each ONU. However, since the optical loss of the signals received by the OLT differs for each ONU, the gain for compensating the loss differs for each ONU. Thus, it is difficult to keep the power within a given dynamic range by the optical post-amplifier.